Ignition coils typically contain a variety of internal components constructed of a variety of different materials. In general, ignition coils typically contain a core assembly constructed of steel lamination stacks upon which the primary and secondary coils of a coil assembly are mounted. The primary and secondary coils typically include a plastic bobbin which is wound with copper wire. The core assembly includes an outer steel lamination stack that engages an inner lamination stack and extends around the coil assembly. A plastic housing is provided to enclose these aforementioned components. An encapsulate such as a thermosetting resin is poured into the housing to fill all gaps that surround the entire assembly.
The lamination stacks forming the core are typically made of silicon steel, and are sufficiently sized to carry the magnetic flux generated by the ignition coil. The other components of the ignition coil, namely the encapsulate and the coil assembly, are made of plastics and resins which have a coefficient of thermal expansion (CTE) 2 to 5 times higher than the steel laminations, while the copper has a CTE about 1.5 times higher than steel. Unfortunately, this mismatch in CTE's can cause cracking in the encapsulate resin surrounding the steel lamination stacks, which can propagate into the secondary windings. With such cracks, the ignition coil can experience internal dielectric failure. Since the secondary windings can carry charges up to 35 K volts, and the lamination stacks have voltage near ground potential, there remains a potential for dielectric breakdown from the secondary windings to the steel laminations.
Accordingly, there exists a need to provide an ignition coil which minimizes the propagation of cracks in the encapsulating resin, thereby reducing the potential for internal dielectric failure.